How valid is the author's interpretation?

The other day I posted some thoughts on an author's authority of interpretation over her own fic. Several people replied, and I found myself getting so excited by the whole thread that I decided to make a new post with all my responses and further musings.

Because, as I have discovered, I actually feel quite passionate about this!

WARNING: major late-night unrevised ramblings behind cut, and lots of 'em.

( why i believe that the author's interpretation is no more valid than anybody else's )